High frequency torch discharge plasma generator provided with single electrode of aluminum



C. 27, 1970 MANABU YAMAMOTO ETAL 5,536,948

HIGH FREQUENCY TORCH DISCHARGE PLASMA GENERATOR PROVIDED WITH SINGLEELECTRODE OF ALUMINUM Flled Jan. 27, 1967 2 Sheets-Sheet 1 F|G.I F|G.5FIG.6

HIGH FREQUENCY TORCH DISCHARGE DISCHARGE-MAIN TAINING GAS-NITROGENELECTRODE-ALUMINIUM (WATER-COOLING) DIRECT CURRENT ARC DISCHARGEDISCHARGE-MAINTAINING GAS-NITROGEN INVENTORS fa/nag, Yamako mgl BY 1754iE ct. 27., 1970 l MANABU YAMAMOTO EVAL 3,536,948

HIGH FREQUENCY TORCH DISCHARGE PLASMA GENERATOR SINGLE ELECTRCDE OEALUMINUM PROVIDED WITH 1967 2 Sheets-Sheet 2 Filed Jan. 27,

F l G. 2 F l G. 3

mi A7 48 6 M/ m4. m. m w M w wnO/Jh OO PO 6 ./.Vm 44 44 3 3 .r ww w w ww A t L L L J A .A AZ E Y zmwznmmohmd B oA O. 4 7 2 3 U C l 48` 8 4 48 39 TO n 2 OO 6 4 44 3 W WW W United States Patent U.S. Cl. 313-231 2Claims ABSTRACT F THE DISCLOSURE A high frequency torch discharge plasmagenerator of the present invention is provided with a single electrodeof aluminum which is least erosive by high temperature at the time ofapplying high frequency, high voltage to the electrode. The apparatus isconstructed with a single electrode held at the tip of a supporterhaving inlet and outlet for cooling water to prevent overheating of theelectrode and a shielding tube made of high refractory and insulatingmaterial having inlet for a discharge-maintaining gas.

This application is a continuation-in-part of prior application Ser. No.293,532 filed on July 8, 1963, now abandoned, and entitled HighFrequency Torch Discharge Generator.

The present invention relates to an improvement in high frequency torchdischarge plasma generator provided with a single electrode.

An essential object of the present invention is to provide a highfrequency torch discharge plasma generator provided with only oneelectrode composed substantially of aluminum and having variousadvantages such that said single electrode is not eroded at all and thedischarge is maintained at very stable state for a long period of timeand is not accompanied by any noise.

Said object and the other objects of this invention have been attainedby using aluminum as the material of the single electrode of a highfrequency torch discharge plasma generator.

The nature, objects and advantages of the present invention will becomemore apparent by reference to the following description when taken inconjunction with the accompanying drawings in which:

FIG. 1 is a sectional elevation for showing one preferred embodiment ofthe high frequency torch discharge plasma generator according to thepresent invention;

FIGS. 2-4 show, respectively, spectra of discharge plasmas of highfrequency discharge plasma generators, wherein FIG. 2 relates to thecase of using an electrode made of tungsten, FIG. 3 relates to the caseof using an electrode made of copper, and FIG. 4 relates to the case ofusing a tungsten electrode;

FIG. S shows a spectrum of the discharge plasma generated by a highfrequency torch discharge plasma gen erator according to this invention,in which single aluminum electrode is used; and

FIG. 6 shows one example of the spectrum of the plasma produced by D.C.arc discharge plasma generator.

A high frequency torch discharge plasma can be generated by disposing anelectrode applied with a high voltage of a high frequency in a gasstream. Generally, in such a discharge plasma generator as describedabove, the most important problem consists in electrode erosion. Thatis,

the electrode is placed under an extremely erosive condition on accountof it contacting a high temperature ionized gas, and when any electrodeerosion occurs, it may cause the electric discharge to be unstable.Accordingly, for example, in the case of plasma jet generator utilizinga direct current arc discharge, a rare gas is used as thedischarge-maintaining gas, a tungsten electrode is used as thestick-shaped cathode, and a water-cooled copper electrode is used as thenozzle-shaped anode in order to protect the electrodes from theirerosion. ln the D.C. arc discharge plasma generator of the conventionaltype, wherein two mutually opposing electrodes are provided, and adirect current voltage is applied across these two electrodes so as toconduct a D.C. arc discharge therebetween, it is absolutely necessary totake measures to avoid erosion of the electrodes, which contact thishigh temperature gas, by means of, for example, using the so-callednon-consumable metal of high melting point such as tungsten, etc. as theelectrode material, or Water-cooling an electrode made of metal ofrelatively high melting point. When a low melting metal such as aluminumis used as the electrode material, the electrode is intensely worn outwith the result that it is impossible to maintain stable dischargeplasma for a long period of time. In fact, in the D.C. arc dischargeplasma generator, even when high melting metal such as tungsten,molybdenum, or the like is used for a cathode and an anode (nozzleelectrode), there unavoidably occurs wear of these electrodes more orless. And, by this wear of the electrodes, splashes of the electrodematerial mix in the plasma, which are excited to luminesce in the plasmaand cause mal-effect to the luminescent spectrum. Thus, in the D C. arcVdischarge plasma generator, there has been adopted a method to minimizethe erosion or consumption of the electrodes as far as possible by useof high melting metal as the electrode material. However, according tovarious experiments by the inventors, it h-as been found that theabove-mentioned method is not effective for the high frequency torchdischarge. That is to say, when the high frequency torch dischargeplasma is subjected to a spectroscopic investigation, many spectrallines of tungsten such as shown in FIG. 2 and FIG. 4 would appear evenwhen a tungsten having an extremely high melting point is used as theconstituting material of the single electrode of said high frequencytorch discharge plasma generator. On the other hand, even when copperhaving a high heat conductivity is used as the single electrode metaland sufficient water cooling is effected to the copper electrode, manyspectral lines of copper appear as shown in FIG. 3. In the highfrequency torch discharge plasma generator, therefore, perfectprevention of the electrode from being Worn out is impossible, even ifthe single electrode is constructed with a high melting metal. Accordingto these facts, it has become clear that there is an essentialdifference between the high frequency torch discharge and the directcurrent (D C.) arc discharge, and also between their discharge phenomenathemselves. Generally, the plasma produced by the direct current arcdischarge assumes a state near thermal equilibrium and the gastemperature and the electron and ion temperature in the said D.C. arcdischarge plasma are of from 7,000 K. to 8,000" K., but the gastemperature in the high frequency torch discharge plasma is remarkablylow. This fact seems to be correct, because there was a clear differenceobtained by spectroscopic investigation in spectrum appearance in bothcases of the direct current arc discharge and high frequency torchdischarge which were carried out by the use of the same nitrogen as thedischarge-maintaining gas. When such a spectrum produced by D.C. arcdischarge as shown in FIG. 6 and such a spectrum produced by highfrequency torch discharge as shown in FIG. 5 are compared with eachother, the intensity of the nitrogen molecular band in the former casedecreases more suddenly than that of the latter case. According to thisfact, it is true that the gas temperature in the case of high frequencytorch discharge plasma is lower than the gas temperature in the case ofD.C. arc discharge plasma. However, in the case of high frequencydischarge, as shown in FIGS. 2 and 4, tungsten having a very lowluminous characteristic can be easily excited. Accordingly, theexcitation temperature in the case of high frequency torch dischargeplasma can be guessed to relatively high.

Thus, in the D.C. arc discharge plasma generator, consumption of theelectrode becomes less as the melting point of the electrode material ishigh, while the consumption thereof becomes great as the melting pointof the electrode material is low. However, on the other hand, thepresent inventors have confirmed through their experiments that in thehigh frequency torch discharge plasma generator having only oneelectrode, there is no relation at all between the melting point of theelectrode material and the degree of its consumption, so that theelectrode cannot always be prevented from being consumed even by use ofmaterial having a high melting point. Based on this discovery, theinventors conducted further experiments to prove that, when aluminum isused as the electrode material for the high frequency torch dischargeplasma generator, consumption of the electrode can be perfectly removed.

Therefore, the present invention is to provide a high frequency torchdischarge plasma generator provided with only one electrode composedsubstantially of aluminum and having various advantages such that saidelectrode is not eroded at all and the discharge is maintained at a verystable state for a long period of time and is not accompanied by anynoise.

This invention is based on a new discovery that aluminum is the mostappropriate material for the single electrode of the high frequencytorch discharge plasma generator, said discovery having been obtained asa result of various experiments relating to various electrode materials.In the high frequency torch discharge plasma generator utilizing awater-cooled aluminum electrode, no erosion of the electrode occurs andit can be confirmed by a spectroscopic investigation that no aluminumline appears as shown in FIG. 5.

In FIG. Il is shown an embodiment of a high frequency torch dischargeplasma generator according to the present invention, said generatorcomprising a single aluminum electrode 1, an electrode supporter 2, ahigh frequency power source 3, a shielding tube 4. The shielding tube 4is made of a highly refractory as well as electrically insulatingsubstance such as, for instance, quartz, etc. This tube is `disposedcoaxially about the supporter 2 and the electrode 1 fitted at the tipthereof, the rear end (the end to the side of the support 2) of whichbeing closed and the front end (the end to the side of the electrode 1)of which fbeing left open. At the rear end of this shielding tube 4,there is provided an inlet 6 for a discharge-maintaining gas. Thesupporter 2 is constructed with an electrically conductive material suchas, for example, copper in a form of double tube, thereby to provide aflow path of the cooling water leading to an outlet 8.

In the actual operation, a discharge-maintaining gas such as nitrogen,argon, etc. is introduced from the gas inlet 6 into the supporter Z insuch manner that the introduced gas stream may flow surrounding theelectrode 1 and jet out to the external atmosphere from the open endthereof. In this state, when a high frequency, high voltage is appliedto the tip of the electrode by the high frequency power source, highfrequency torch discharge is generated within the discharge-maintaininggas surrounding the electrode 1. By this discharge, thedischarge-maintaining gas is excited or ionized to generate highfrequency torch discharge plasma.

In this case, cooling water may be introduced from the inlet 7 toprevent overheat of the electrode 1. It is to be noted that, in FIG. l,a gas inlet means to be connected to the gas inlet 6 and a cooling watersupply means to be connected to the cooling water inlet 7 are all leftout.

In FIG. l, one output terminal of the high frequency power source 3 isconnected to the supporter 2 for the electrode l so as to apply highfrequency, high voltage to the tip of the electrode 1. However, itshould be understood of course that any other expedient is applicable asthe means for impressing high frequency, high voltage on the tip of thiselectrode 1, such as one in which a tank coil is wound around theshielding tube 4, through which a high frequency current is caused toow; or one in which high frequency power is transmitted through acoaxial cable so as to directly supply the high frequency voltage to thealuminum electrode fitted at the tip of the internal conductor of thecoaxial cable; or further one in which a cavity resonator is providedabout the shielding tube 4, and a high frequency power is supplied tothis resonator, thereby inducing high frequency, high voltage at the tipof the electrode "1.

As an example, when high frequency power of 20 mc., 700 w was applied tothe tip of the electrode l, while introducing nitrogen as thedischarge-maintaining gas from the discharge gas inlet 6, the gastemperature of the discharge plasma generated was 4,000-5,000 K. It hasbeen confirmed that this temperature is considerably low in the value incomparison with the gas temperature in the discharge plasma obtainedfrom the D.C. arc discharge of 7,000-8,000 K. It has also been verifiedthat, in the above-described example, the exciting temperature in theplasma reached 7,000-8,000 K, which was considerably higher than that inthe case of the D.C. arc discharge. Also, no spectral line of aluminumcould be detected at all in the light emitted from the discharge plasma.This phenomenon signifies that the aluminum electrode used in the highfrequency torch discharge plasma generator having only one electrode iseroded at its least. Accordingly, a stable plasma can be generated for along period of time, which is advantageous. A particular attentionshould be drawn to the fact that such effect of preventing the electrodefrom erosion can be obtained only in the case of using aluminum as theelectrode material in the high frequency torch discharge plasmagenerator, and that enetrely reverse effect is resulted when aluminum isused as the electrode material for conventional apparatus, in which arcdischarge is performed between two electrodes.

In the high frequency torch discharge plasma generator according to thepresent invention, having only one electrode substantially consisting ofaluminum, there is no electrode erosion, stable operation of a longperiod of time is secured, and no noise occurs. Accordingly, saidgenerator can be utilized for various objects, for example, it can beused as an excitation source for the spectroscopic analysis, and formeasuring excited states of atoms or molecules, and various physicalValues. Particularly, when the present invention is applied to aspectroscopic analysis apparatus, characteristic spectral lines of thespecies can be detected with a high sensitivity by the process whichcomprises coverting the species into powdery state in the case of solidmaterial or into mist state in the case of liquid material and thenintroducing the species of the said converted state into the inlet 6such as shown in FIG. 1 or into a separate species introducing passagetogether with a discharge-maintaining gas; and exciting the saidintroduced mixture in a discharge plasma.

While We have shown and described the preferred embodiment, it Will beapparent that various modifications and changes may be made therein,particularly in the form and relation of parts, without departing fromthe spirit of our invention as set forth in the appended claims.

What We claim is:

1. A high frequency torch discharge plasma generator comprising: asingle aluminum electrode; means to impress a high-frequency voltage onthe tip of said electrode; and means for forming a stream of gascovering the tip of said electrode.

2. A high frequency torch discharge plasma generator comprising: aquartz tube; a single aluminum electrode disposed therein; a highfrequency power source connected to said electrode; an inlet for adischarge maintenance gas, .said gas streaming about the tip of saidelec trode; and cooling means for cooling said electrode.

6 References Cited UNITED STATES PATENTS 2/1963 Gage 313-231 X 8/1966Lai 313-231 X OTHER REFERENCES Roddy & Green, The Radio-Frequency PlasmaTorch, Electronics World, Feburary 1961, pp. 29, 31, and 117.

ROY LAKE, Primary Examiner E. R. LAROCHE, Assistant Examiner U.S. Cl.X.R. 315-111

